1. Field of the Invention
The present invention relates to methods and apparatus for logging a borehole and more particularly to such methods and apparatus in which acoustic energy pulses are transmitted from the central portion of the borehole toward the borehole wall in a plurality of radial directions and at a plurality of different depths thereby generating reflected acoustic pulses from a reflecting boundary which are indicative of the condition of the reflecting boundary.
2. Setting of the Invention
One type of device for logging a borehole to determine the characteristics of the surrounding formation is sometimes referred to as a borehole televiewer. Such a device includes a synchronous alternating current motor. The motor is received in a housing which may be lowered into a well bore. The housing includes a gear box and a number of additional gears and shafts for enabling the motor to rotate a transducer assembly mounted on one of the shafts. As the housing is raised or lowered in a well bore, alternating current is supplied to the motor via a cable which connects the housing to a generator at the earth's surface. As the transducer assembly is rotated by the motor, acoustic pulses emit therefrom along a radial borehole axis thereby helically scanning the borehole with such pulses. A portion of the energy from each acoustic pulse is reflected by a reflecting boundary adjacent the borehole along a radial axis back toward the transducer which detects the reflected energy. The reflecting boundary may be the radially inner surface of the uncased borehole or of the casing. Reflections may also be returned under appropriate conditions from deeper boundaries, for example, from the casing-borehole interface, or from an interface between formations in the ground surrounding the well bore. If logging is conducted in an uncased well, one reflecting boundary will be the borehole wall.
For each reflected acoustic pulse, an electrical pulse is generated which is related to the energy of the reflected acoustic pulse detected by the transducer. These pulses are amplified and applied to a cable for transmission to the earth's surface. At the surface the pulses are typically recorded and thereafter manipulated to produce a display of the formation surrounding the borehole.
Examples of logging systems which operate as described above are disclosed in U.S. Pat. No. 3,728,672 to Dennis et al. and in U.S. Pat. No. 4,463,378 to Rambow. Another prior art device which operates generally in accordance with the above-described principles includes a substantially cylindrical housing having a synchronous alternating current motor received therein. The motor includes an output shaft which is connected by a drive gear to an upper gearing mechanism that includes several gears which may be replaced to change the gearing ratio. Through several additional gears and shafts, the rotational output of the motor shaft is coupled to a transducer drive shaft which is coaxial with the housing.
The lower end of the transducer drive shaft has a gear mounted thereon which is engaged with a lower gearing mechanism that is coupled to a transducer support shaft. A pair of transducers are mounted on the support shaft and are rotated thereon under power of the motor. A rotary transformer is disposed between the support shaft and the radially inner surface of the housing. The transformer includes a rotor mounted on the support shaft and a stator mounted on the radially inner surface of the housing.
A cable connects the stator of the rotary transformer with electronics in the housing which provide signals to and receive signals from the transducers.
In operation, the device is suspended from a cable and lowered into a well bore adjacent an area of interest which is to be scanned. In addition to the support cable, a multi-conductor electrical cable provides communication between the electrical components in the device and circuitry located at the surface.
To initiate borehole scanning, alternating current is applied to the power lines in the electrical cable at the surface which are connected to the motor thus starting rotation of the motor shaft. The power is typically provided by a portable generator and is usually 60 cycle per second alternating current.
The motor drives the upper gearing mechanism which in turn rotates the transducer drive shaft. The transducer drive shaft drives the transducer support shaft via the lower gearing mechanism.
As the support shaft rotates, a periodic electrical pulse is applied to one of the transducers thereby generating an acoustic pulse which is transmitted radially from the device toward the borehole wall adjacent the transducer. The pulse is applied to the transducer via the rotary transformer. Acoustic energy is reflected by a reflecting boundary, such as the interface between the borehole and the surrounding formation, and is detected by the transducer which generates an electrical pulse related to the energy of the reflected acoustic pulse. This pulse appears on wires which connect the transducer to the rotor of the rotary transformer. The pulse is amplified by a downhole amplifier and transmitted to the surface via the electrical cable connecting the device to the circuitry at the surface. Accumulation of data so generated can be used to generate a display of the reflecting boundary.
When it is desired to change the speed of rotation of the transformer support shaft, the device must be raised to the surface to replace gears in the upper gearing mechanism in order to change the gear ratio between the motor shaft and the transducer drive shaft.
Several prior art references illustrate logging systems which operate as explained above. Some of these references show, e.g., FIG. 1 of U.S. Pat. No. 3,728,672 to Dennis et al., schematic illustrations of the prior art which delete the gear boxes and miscellaneous gears between the motor and the transducer assembly; however, such gear boxes and gears are present in the physical embodiments of all prior art devices known to the inventors of the instant invention.
The above-described prior art logging systems suffer from several disadvantages. The synchronous motor is typically powered by a portable generator at the well site and the frequency of the alternating current so generated may vary thereby varying the speed of the motor. The frequency of the electrical pulse which is generated by downhole electronics and applied to the transducer for generating the acoustic pulse may vary due to the typically high downhole temperatures to which the electronics are exposed. With the rotational speed and pulse frequency varying, the total number of acoustic pulses generated per revolution is subject to change and the log generated from such a system may be of uneven resolution. Also, when the number of pulses per revolution varies, error is introduced when the electrical pulses generated from the reflected acoustic pulses are manipulated to calculate volume surrounding the borehole.
As previously mentioned, the prior art devices incorporate a gear box plus additional gears between the shaft of the synchronous motor and the shaft on which the transducer assembly rotates. Such complex gear systems are necessary for three reasons. First, a synchronous motor has a vey low start-up torque and thus requires a fairly high gear ratio in order to begin rotation of the transducer assembly. Secondly, synchronous motors are typically designed to operate at a selected frequency, often 60 cycles per second, and any deviation therefrom substantially reduces the output torque. A synchronous motor which is powered by voltage at 60 cycles per second typically rotates at 1800 or 3600 rpm which would rotate the transducer assembly too rapidly and thus gearing is required to reduce the rotational speed of the transducer assembly. Finally, when it is desired to change the speed of transducer rotation in order to change the resolution or to change the rate at which the transducer is advanced along the borehole while maintaining the same resolution, the tool must be raised to the surface and the gears changed.
In prior art systems which utilize a synchronous motor, the gear system described must be used for the foregoing reasons. However, because of the friction inherent in the gear system and because of the gearing ratios which must be used to enable the motor to start and maintain rotation of the transducer assembly, the maximum rotational speed of the transducer assembly, and thus the maximum speed of advancement along the borehole axis, is subject to an unacceptably low upper limit. Thus, such logging consumes a great deal more time than other forms of borehole logging.
The use of prior art systems incorporating a synchronous motor and a gear system as described above necessitates raising the tool to the surface when it is desired to vary the rate of rotation of the motor. As discussed above, this is done by swapping gears in the tool thereby changing the gear ratio. The tool may then be again lowered into the borehole to commence scanning at the new rotation rate. It would be very desirable to have the capability of varying the rotation rate of the motor from the surface without having to raise the tool to the surface. This would eliminate the time and expense necessary to raise the tool to the surface and again lower the tool in the well bore to the point of interest.
It would also be desirable to be able to vary other operating characteristics of the tool from the surface. For example, rate of acoustic pulse scanning could be varied by varying the frequency at which the transducer firing pulses are generated. Further, it may be necessary or desirable to change the width of such pulses and to change other parameters of the downhole control circuitry. Prior art tools have not provided means for varying such operating characteristics from the earth's surface. As in the case of controlling motor rotation rate, it would be desirable to have the capability to vary such operating characteristics at the surface without having to raise the tool for adjusting the same.
There exists a need for a method and apparatus for generating a signal containing information relating to the state of a borehole of the type described in which the rotational speed of the transducer may be varied from the surface.
There also exists a need for such a method and apparatus in which the maximum rotational speed of the transducer is increased over that of prior art methods and apparatus.
There exists a further need for such a method and apparatus in which the rotational speed of the transducer is synchronized with the rate at which the transducer generates acoustic pulses.
There exists yet another need for such a method and apparatus in which operating characteristics of the downhole portion of the system, e.g., acoustic pulse width, acoustic pulse frequency, amplifier gain, etc., may be both monitored and adjusted at the surface.